1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly, to an image forming apparatus, which can prevent inferior image quality due to current leakage by an optimized installation of an eraser.
2. Description of the Related Art
FIG. 1 schematically illustrates a conventional tandem-type electro-photographic image forming apparatus.
Referring to FIG. 1, a conventional image forming apparatus includes a plurality of image bearing members 10, on which an image is formed by a developing unit (not illustrated); a transfer unit 20 which transfers a visible image formed on each image bearing member 10 onto a printing medium M; and a fixing unit 30 which fixes an image on the printing medium M by heat and pressure.
The transfer unit 20 includes a plurality of transfer rollers 21 disposed opposite to their respective image bearing member 10, and a transfer belt 25 feeding the printing medium M while passing between the image bearing member 10 and the transfer rollers 21. The transfer unit 20 directly transfers a toner on each image bearing member 10 to the printing medium M by a transfer voltage applied to the transfer roller 21 and having a polarity opposite to that of a voltage applied to the image bearing members 10. When the transfer voltage is applied to the transfer roller 21, the transfer voltage is also applied to the printing medium M.
When the printing medium M passes through a transfer nip formed between each image bearing member 10 and each transfer roller 21, gap discharge (that is, printing medium detachment discharge) is generated in the front or the rear of the transfer nip, particularly, in an area A in front of the transfer nip as illustrated in FIG. 1, due to electric potential difference between the image bearing member 10 and the printing medium M, thereby causing scattering in an image formed by a toner.
In order to prevent the image scattering due to the gap discharge in the area A, a conventional image forming apparatus includes an eraser 40, as illustrated in FIG. 2. The eraser 40 erases a voltage applied to the printing medium M to reduce electric potential difference between the image bearing member 10 and the printing medium M, thereby restraining the gap discharge.
Generally, current flowing between an eraser and the ground is less than 0.1-1.0 μA. Thus, when the eraser 40 is disposed near the transfer roller 21, a transfer current may not flow from the transfer roller 21 toward the printing medium M due to high resistance of the printing medium M under a certain condition, and thus, current leakage may occur toward the eraser 40 via the transfer belt 25, thereby causing inferior image quality.
Hence, an eraser should be installed in such a position as to prevent current leakage toward the eraser.
Further, in a conventional electro-photographic image forming apparatus, when a printing medium of high resistance is used under a low-humidity environment or during duplex printing, resistance of a transfer belt increases. Thus, the amount of electric charges accumulated in the transfer belt is rapidly increased. As a result, it is difficult to form an electric field only by applying a voltage to a pressing roller (35 in FIG. 1) of the fixing unit 30, thereby causing an electrostatic offset. Thus, when a printing medium with toner being transferred is fixed in the electrified state, inferior image quality is likely to occur due to the electrostatic offset.